In a case where an inductive load such as a solenoid valve and a motor is driven on or off, an induced voltage is generated by energy accumulated while the inductive load is off. An active clamp circuit is used to protect a switch element and the like from the induced voltage. The active clamp circuit clamps the induced voltage at a predetermined value to absorb the energy accumulated in the inductive load.
However, a clamp voltage, which depends on an amount of current that flows through the active clamp circuit, is high at the start of the clamp operation and reduces over time. For example, the clamp voltage changes due to a fluctuation of a current-voltage characteristic and the like of the zener diode when the induced voltage is clamped by breaking down a zener diode.
An operation period of active clamp is a wasteful time period during which the inductive load cannot be driven and thus is preferably short.
Moreover, while the active clamp circuit is not supplied with a power supply voltage, the active clamp circuit does not operate and thus the switch element and the like may be broken by electrostatic discharge (ESD).